The present invention relates in general to a motor drive technique, and pertains, more particularly to a power transistor drive circuit usually used in an inverter type configuration in a multiphase motor drive. The base drive circuit of the invention operates to derive maximum power transistor protection, at the same time permitting maximum performance. Optimum load drive is possible but is accomplished without fear of component overload.
In an inverter drive for an AC motor there are typically provided power transistors associated with each phase. The output power transistor is switched on a selective basis from a high voltage non-conduction state, to a low voltage conduction state in which there is a large collector current. This switching from a high voltage to a low voltage state occurs as rapidly as possible. During this switching action there is generally a much larger base current provided during the initial portion of the conduction cycle than during the remainder of the cycle. Conditions may exist wherein the load becomes shorted in which case the base drive causes excessive collector current to flow at the same time that there is a full supply voltage across the transistor. Under such conditions the collector current is usually sensed and is limited with the transistor then being turned off, preferably within several microseconds.
Accordingly, it is an object of the present invention to provide an improved transistor drive circuit, referred to herein as a base drive circuit, for controlling the operation of a power switching transistor. The base drive circuit isolates and amplifies an input control signal to a sufficient level to operate the power switching transistor.
Another object of the present invention is to provide an improved base drive circuit that also provides for protection of the power transistor so that the transistor is not destroyed during operation.
Still another object of the present invention is to provide a base drive circuit that senses not one but preferably two parameters of the power transistor to derive maximum protection and allow maximum performance substantially simultaneously.